Biological Membranes

Biological Membranes Notes

Introduction

Founded in 1911, the Biochemical Society established the Biochemistry Across the School Curriculum Group (BASC) in 1985.
BASC creates booklets to aid teachers with syllabuses that contain high biochemical content.
Booklets cover topics like: Essential Chemistry for Biochemistry, Nucleic Acids, Enzymes, Metabolism, Immunology, Photosynthesis, and Recombinant DNA Technology.
Biological membranes are crucial for life. Provide cell boundaries (plasma membranes) and internal compartments (organelles).
Membranes are selectively permeable which regulates the movement of substances in and out of cells, and controls flow of information between cells.
Membranes are involved in energy capture/release (photosynthesis, oxidative phosphorylation).

Membrane Components

Biological membranes consist of lipids, proteins, and sugars.
Lipids form a two-layered sheet with embedded proteins.
Structure is fluid, allowing lipid and protein movement within the membrane plane.
Lipids and proteins are held together by non-covalent interactions.
Sugars attach to lipids/proteins via covalent bonds, found on the outer surface of the plasma membrane.

Types of Lipids

Lipids are insoluble in water but soluble in organic solvents (propanone, ethanol, trichloromethane, ethoxyethane, light petroleum, etc.).
Three major types exist in biological membranes: phospholipids, glycolipids, and cholesterol.

Phospholipids

Most common type consists of glycerol linked to two fatty acid chains, phosphate, and choline (Figure 2).
Fatty acid chains contain 14-24 carbon atoms, with one usually unsaturated (containing 1-4 cis double bonds).
Double bonds create bends in the fatty acid chain.
Lipids of this type are called glycerophospholipids. The three major glycerophospholipids contain choline, serine, or ethanolamine attached to the phosphate.
Serine: $HOCH2CH(COO^-)NH3^+$
Ethanolamine: $HOCH2CH2NH_3^+$
Another type of phospholipid contains sphingosine instead of glycerol. Sphingomyelin is a common example, containing choline attached to the phosphate.

Glycolipids

Glycolipid molecules contain either glycerol or sphingosine linked to fatty acid chains (Figure 3).
Differ from phospholipids by having a sugar (glucose or galactose) instead of a phosphate-containing head.
Animal membranes usually contain sphingosine, while bacterial and plant membranes mainly contain glycerol.
Glycolipids are found on the outer surface of plasma membranes with sugars exposed at the cell surface.

Cholesterol

Cholesterol has a different structure from phospholipids and glycolipids.
Contains a four-ring steroid structure, a short hydrocarbon side-chain, and a hydroxy group (Figure 4).
Found in mammalian membranes and mycoplasmas (small bacteria without a cell wall).
Not usually found in bacterial or plant membranes.

Amphipathic Nature of Membrane Lipids

Membrane lipids are amphipathic which means they have a hydrophilic (water-loving, polar) region and a hydrophobic (water-fearing, non-polar) region.
Phospholipids and glycolipids have a hydrophilic head and two hydrophobic tails.
In water, they spontaneously form bilayers with hydrophobic tails sandwiched between hydrophilic heads (Figure 5).
Bilayers form because hydrocarbon tails stay away from water and are squeezed together by water molecules.
Bilayers close to form sealed compartments called liposomes (Figure 6).
Liposomes are useful model membranes for research and drug delivery.
Cholesterol is also amphipathic, with hydrophobic rings and side-chain, and a hydrophilic hydroxy group.
Can be incorporated into phospholipid bilayers (Figure 7) but cannot form a bilayer alone.

Membrane Proteins

Membrane proteins carry out specific functions.
The amount and types of protein vary from membrane to membrane.
Metabolically active membranes contain more protein.
Proteins associate with lipid bilayers in different ways (Figure 8):
- Transmembrane proteins: extend across the lipid bilayer, containing hydrophobic regions that interact with the hydrophobic tails of the phospholipids. These regions are often helical, forming rigid tubes studded with hydrophobic amino acid side-chains. There may be one or more such regions in a transmembrane protein. The hydrophilic regions of these proteins extend from both sides of the membrane. Some transmembrane proteins may be covalently linked to a fatty acid chain which is inserted into the phospholipid bilayer (Figure 9a).
- Anchored proteins: intracellular proteins covalently attached to the inner surface of the membrane via a fatty acid chain or phospholipid (Figure 9b). Also called integral membrane proteins removed by treatments that disrupt the membrane.
  - Examples: cholinesterase (synapses), G-proteins (sending messages across membranes).
- Peripheral membrane proteins: weakly bound to the membrane surface via non-covalent interactions with other membrane proteins (Figure 9c).
  - Removed by mild treatments (altering pH or ionic strength) leave membrane intact.
  - Example: cytochrome c (inner mitochondrial membrane).

Glycoproteins

Most plasma membrane proteins exposed to the cell surface have covalently linked sugars (Figure 10).
Sugars are linked to the $–CONH_2$ side-chain of asparagine or to the $–OH$ in the side-chains of serine or threonine.
Sugars are short, branched chains containing 4-12 residues.

Two-Dimensional Fluids

Lipid bilayers are two-dimensional fluids where lipids and proteins can move past each other (Figure 11).
Phospholipid lateral diffusion is rapid: diffusion coefficient is about $1 µm^2⋅s^{-1}$ . Move around perimeter of a human red blood cell in 12 s.
Lipid molecules rotate rapidly along their head-to-tail axes, flexible tails can wave about.
Membrane proteins have variable mobility.
- Rhodopsin (retina) is fairly mobile, with a diffusion coefficient of $0.4 µm^2⋅s^{-1}$ .
- Fibronectin (binds cells) is slower, with a diffusion coefficient of about $100 nm^2⋅s^{-1}$ . It is about 40000 times slower than rhodopsin.

Separate Layers

Lipid molecules rarely move from one monolayer to the opposite one; lipid composition varies between layers.
Transverse diffusion or flip-flop of phospholipids is rare (Figure 12) due to the polar head needing to penetrate the hydrophobic core.
Flip-flop occurs once in several hours.
Phospholipid translocators (flippases) in the endoplasmic reticulum enable rapid flip-flop of particular lipids across the bilayer.
Proteins are less likely to flip-flop as it has not been observed.

Membrane Fluidity

Fluidity depends on temperature, fatty acid composition, and cholesterol content.
At low temperatures, hydrocarbon tails pack together to form a rigid gel state.
As temperature increases, the bilayer melts into a more disordered, fluid liquid state.
Transition temperature is the melting temperature of the lipid bilayer (typically $10–40 °C$ for biological membranes).
Shorter lipid tails and double bonds increase fluidity (lower transition temperature).
Eukaryotic animal membranes contain roughly one cholesterol molecule for every two phospholipid molecules.
Cholesterol fits between phospholipids, stiffening lipid tails and reducing fluidity.

Membrane Carbohydrates

Sugar residues make up less than 10% of membrane weight.
They vary in structures in short chains and give cell types distinguishing features, which can be involved in cell recognition.
Red blood cells have branched carbohydrates for blood-group antigens (A, B, O).
Cell-surface differences cause specificity of action with hormones, drugs, viruses, or bacteria.

Membrane Structure Summary

The fluid mosaic model (Singer and Nicolson, 1972) depicts the membrane as a fluid lipid-protein mixture with molecules moving in two dimensions.
Sugar chains stick out from one surface.
Biological membranes are lipid–protein–sugar ‘sheets’:
- Lipids provide permeability barrier and structural integrity.
- Proteins carry out specific functions.
- Sugars provide distinctive appearance.

Evidence for Membrane Structure

Lipid Bilayer Proof

Gorter and Grendel (1925) extracted membrane lipids from red blood cells. Formed a continuous macromolecular layer which occupied an area of about twice the surface area of the original red blood cells, showing lipids formed a bilayer.
X-ray diffraction shows low electron density in the membrane interior and high density at the edges, supporting the bilayer structure.
Freeze-fracture electron microscopy confirms the bilayer structure.
X-ray diffraction and electron microscope studies show that many natural membranes are approximately $5 nm$ thick.

Membrane protein Visualization

Membrane proteins can be visualized and measured by SDS/polyacrylamide-gel electrophoresis (Figure 13).
SDS (sodium dodecyl sulphate) is an amphipathic molecule with a negatively charged hydrophilic head and a hydrophobic tail that disrupts membrane structures and binds to hydrophobic regions of membrane proteins, making them have an overall negative charge, and the gel acts as a sieve so that small proteins move furthest through the gel, while large ones are retarded and stay near the point of application. The distance moved is inversely proportional to the logarithm of the molecular mass.
Freeze-fracture techniques provided the first evidence for integral membrane proteins.
Inner faces of split monolayers reveal globular particles (5.0–8.5 nm in diameter) that are membrane proteins.

Bilayer Mobility Demonstration

Cell fusion experiments (Frye and Edidin, 1970) showed protein mobility (fluid bilayer).
Human and mouse cells were labeled with red and green fluorescent dyes and fused, the labels were distributed randomly over the surface after 40min demonstrating that integral proteins diffuse freely in the lipid bilayer.
Fluorescence photobleaching recovery also showed lipid/protein mobility.
A cell-surface component is labelled with a fluorescent dye. A small area (say $3 µm^2$ ) of cell surface is viewed under a fluorescence microscope, and the fluorescent molecules in this region are destroyed with a very narrow beam of laser light.
Artificial bilayers (liposomes, black membranes) help measure mobility and permeability.
Spin-labeled phospholipids reveal rapid lipid movement but slow flip-flop.

Surface Sugars Analysis

Lectins (carbohydrate-binding proteins) reveal carbohydrates only on the outer surface of plasma membranes.
Lectins, treated with ferritin such that they can be seen under the electron microscope, bind to specific sugar sequences on glycolipids and glycoproteins.

Membrane synthesis

New Membranes

Membranes grow by the expansion of pre-existing membranes.
In prokaryotes, membrane components are synthesized on the inner surface of the plasma membrane; in eukaryotes they are synthesized on the ‘outer’ (cytosolic) face of the endoplasmic reticulum (ER).
The flow of membranes in eukaryotic cells is from ER to Golgi complex to plasma membrane to cell exterior — unless special signals direct the flow to other organelles, to vesicles for secretion under hormonal or nervous control, or even back to the ER (Figure 14).

Membrane lipids synthesis

Phospholipid molecules are assembled step-by-step (Figure 15).
Two activated fatty acids attach to cytosolic glycerol phosphate.
The resulting amphipathic molecule (diacylglycerol phosphate) embeds in the ER membrane by its two hydrocarbon tails.
Phosphate is replaced by the polar head-group (for example, phosphate-choline) to form a membrane phospholipid.
Enzymes involved are integral proteins in the ER membrane, their active sites face the cytosol.
Reactions happen in the outer monolayer of the ER membrane.
Flippases transfer newly formed phospholipids to the opposite monolayer (Figure 16).
Flippases don't bind well with all phospholipids which results in the membrane monolayers having different distributions of phospholipid.
Final stage: phospholipids are transferred from the ER membrane to other cellular membranes.
Membrane vesicles bud off from the ER and fuse with membranes of the Golgi complex. Other vesicles bud off from the Golgi complex and fuse with membranes of other organelles (such as lysosomes) or with the plasma membrane. The membranes of transport vesicles contain phospholipid, glycolipid and cholesterol, as well as membrane proteins.
Organelles, such as mitochondria, acquire proteins from the ER by a different mechanism.
Water-soluble carrier proteins called phospholipid-exchange proteins (or phospholipid-transfer proteins) remove phospholipids from the ER membrane and deposit them in the membranes of the appropriate organelles.
The ER also synthesizes cholesterol and the sphingosine–fatty acid compound that forms the basis of sphingolipids.
Sugars are added to form glycolipids and phosphate-choline is added to form the phospholipid sphingomyelin, in the Golgi complex. The glycolipids remain on the inner bilayer of the Golgi complex and are taken to the plasma membrane by transport vesicles. These vesicles may be the same ones that take secretory proteins and integral membrane proteins to the plasma membrane.

Membrane protein synthesis

Synthesis of membrane proteins begins on ribosomes in the cytosol, but the first stretch of protein synthesized (the amino terminus) contains a signal that directs the ribosome to the ER. This signal is a sequence of 15–20 amino acids, of which about half are hydrophobic.
The signal is recognized by a signal recognition particle (SRP), a complex of protein with RNA which brings the growing protein chain and attached ribosome to the ER and binds it to an SRP receptor in the ER membrane. The SRP is released and the growing protein chain is directed into a protein-conducting channel which penetrates the ER membrane so that the protein, as it is synthesized, emerges in the ER lumen.
The signal sequence is sometimes called a start-transfer signal because it directs the protein to pass through the ER membrane. Once inside the lumen, this signal sequence is removed by a membrane-bound peptidase: a soluble protein is formed which will eventually be secreted from the cell (e.g. digestive enzymes secreted into the gut).
Proteins destined to become integral transmembrane proteins contain one or more stop-transfer signals, each of which is a sequence of about 20 hydrophobic amino acids within the protein chain. Such signals arrest the passage of the growing protein chain through the ER membrane, so the protein remains embedded in the membrane.
Sugars are attached to membrane proteins during their synthesis in the ER.
Transport vesicles carry proteins in their membranes from the ER to the Golgi complex where they are further modified (for example, by the addition of further sugars), sorted and sent to their final destination. Unless directed otherwise, proteins will be taken by secretory vesicles to the plasma membrane. Therefore, proteins that are synthesized in cells contain signals that determine their ultimate destinations.

Small molecule transportation across membranes

Selectively permeable bilayers

Not all molecules can cross membranes equally well: membranes are selectively permeable.
Small hydrophobic molecules can readily cross phospholipid bilayers by dissolving in the hydrophobic core by simple diffusion. The smaller the molecule and the more fat-soluble they are, the faster it will penetrate the membrane. Thus small non-polar molecules such as $O2$ and $N2$ , and uncharged polar molecules such as $CO_2$ , ethanol or urea, can rapidly cross lipid bilayers. They cross a $10 nm$ bilayer in seconds.
The rate of diffusion is described by Fick’s law.
- Rate is directly proportional to the concentration difference $(Co – Ci)$ , membrane area $(A)$ , and permeability coefficient $(P)$ , which is inversely proportional to membrane thickness $(d)$ .
- Formula: $Rate = –P(Co – Ci)$
- $P = frac{KD}{d}$
- Where:
 - $C_o$ = concentration outside
 - $C_i$ = concentration inside.
 - $P$ = permeability coefficient
 - $K$ = Partition Coefficient.
 - $D$ = diffusion coefficient
 - $d$ = membrane thickness.
Large, uncharged polar molecules like glucose may take hours to pass through a $10 nm$ bilayer.
Ions like $Na^+$ and $Cl^−$ are much less likely to get across, taking weeks to penetrate a $10 nm$ bilayer.
Special transport mechanisms are needed to get these molecules across membranes.

Water molecules

Water molecules can cross lipid bilayers rapidly, passing through a $10 nm$ bilayer in about one millisecond, even though it is a polar molecule.
Water moves through biological membranes about $10^5$ times faster than do glucose molecules, and $10^{10}$ times faster than do $Na^+$ and $K^+$ ions.
It is suggested this is due to
- The water molecule is very small, so its size helps it to cross the membrane.
- Its concentration $(55.5 mol⋅dm^{-3})$ is very high.
- Its dipolar nature may help it to cross the charged lipid head region of the bilayer.
- Water dissolves to a very slight extent in the hydrophobic core of the membrane; there is about one water molecule for every 20–40 lipid molecules.

Downhill and uphill movement

Molecules move from a region of high concentration to a region of low concentration downhill.
Energy can be obtained from the downhill flow; the amount (known as the free energy change, ∆G) is proportional to the concentration difference.
Energy is needed to make molecules move uphill, from a low concentration to a high concentration.
The above applies to charged molecules (i.e. to ions) in addition to concentration gradient, there is also electrical potential difference across the membrane. This is because the inside of many cells is electrically negative compared with the outside, so the entry of positive ions is favoured, whereas that of negative ions is opposed.
For a positive ion moving into the cell, the strength of the inward attraction will depend on the concentration gradient. But it will also depend on the number of charges on the ion (Z): an ion with two positive charges will be attracted more strongly than an ion with only one positive charge. The movement of ions will also depend on the size of the potential difference across the membrane (∆Ψ): the more negative the inside of the cell, the greater will be the attraction experienced by an entering positive ion.
The hill for charged molecules is, therefore, a combination of concentration gradient and electrical gradient — the electrochemical gradient.

Membrane Transport Proteins

Ions , sugars and amino acids cannot diffuse across phospholipid bilayers fast enough to meet the cells’ needs.
They are transported by integral membrane proteins called transport proteins, and the process is sometimes called mediated transport. There are two types of transport protein: channels and carriers.

Channel Proteins

Channel proteins form water-filled pores across the bilayer.
Ions flow downhill, down their electrochemical gradient, at a rate of about $10^8$ ions per second.
Channels are built from four, five or six protein subunits, assembled to form a pore.
The amino acid side-chains which line the pore will determine its selectivity: pores that admit positive ions are lined with negative side-chains, whereas pores that admit negative ions are lined with positive side-chains.
The size of ion admitted is determined by the diameter of the narrowest part of the pore.
The shape of the protein subunits, and therefore whether the channel is closed or open, is affected by the binding of signal molecules to the subunits, or even by a change in membrane potential.
Channels are gated, either ligand-gated (if they are opened by the binding of a signal molecule) or voltage-gated (if they are opened by a change in membrane potential).

Carrier Proteins

Carrier or transporter proteins bind specific molecules or ions and transfer them across the membrane. Transports one molecule of their class, the rest are rejected (e.g. the glucose transporter transports glucose but not other sugars).
The binding of the substance causes the carrier to change shape:
- Substance is exposed first on one side of the membrane , then the other.
- The affinity of the binding site for the bound substance decreases, so the transported molecule is released on the opposite side of the membrane.
The maximal transport rate for carriers is about 10 molecules per second: they are much slower than channels.
Some carrier proteins transport only one solute across the membrane (uniports). Others transport two solutes at the same time(co-transporters):
- Symport: two solutes are transported in the same direction.
- Antiport: two solutes are transported in opposite directions.

Passive and Active Transport

All channel proteins and many carrier protein transfer molecules across the membrane downhill.
This is called passive transport, or facilitated diffusion, because no input of energy is needed.
Facilitated diffusion differs from simple diffusion in that it is selective and saturable.
Cells also have transport proteins that transfer solutes across the membrane uphill, against their electrochemical gradient. This process is called active transport because an input of energy is needed to bring it about.
It is always done by carrier proteins, not by channels.
The energy to drive active transport may come from a number of sources. The most common source is the hydrolysis of ATP. Others include light energy and the energy stored in ion gradients.
The original ion gradient arises from a primary active transport process which uses a direct source of energy, such as ATP or light. The transport that depends on the ion gradient (which in turn has to be generated by primary active transport) is referred to as secondary active transport.

Examples of membrane pumps

ATP driven pump

All animal cells actively pump $Na^+$ ions out and pump $K^+$ ions in. These two transport processes are carried out by the enzyme $Na^+/K^+-$ exchanging ATPase, also called the sodium pump.
The sodium pump is an integral protein of the plasma membrane. It is a tetramer of two types of subunit, one large and one small:
- The large subunit contains the ATP-binding site and is involved in ion transport.
- The small subunit has sugars on its extracellular surface, but its function is not known.
- Mechanism:
  - $Na^+$ binds on the cytocolic side which triggers phoshorylation catalyzed by ATP.
  - Phosphorylation releases Na ions due to the subunit shape change.
  - $K^+$ binds which triggers subunit dephosphorylation.
  - Dephosphorylation causes the subunit to change the shapre back to its original shape, so the binding site faces the inside.
  - $K^+$ ions are released and ATP can be bound again, ready to repeat the cycle.
The gastric enzyme $H^+/K^+-exchanging ATPase$ uses ATP hydrolysis to pump $H^+$ ions out of the cells and into the stomach interior, in exchange for $K^+$ ions.
The muscle enzyme $Ca^{2+}-transporting ATPase$ pumps $Ca^{2+}$ ions from the cytosol into the sarcoplasmic reticulum during relaxation of muscle cells.

Light driven pump

A purple bacterium called Halobacterium halobium lives in high-salt environments and contains, in its plasma membrane, a pump that uses light energy to pump $H^+$ ions out of the cell.
The resulting proton gradient is then used to synthesize ATP. The pump consists of an integral membrane protein with a covalently attached molecule of retinal, hence the purple color.
Retinal is sensitive to light; the absorption of light will cause its straight hydrocarbon tail to bend (as a trans double bond is isomerized to cis).

Ion gradient driver pump

Active transport can be driven by gradients of $H^+$ or $Na^+$ ions which, in turn, have been generated by another active transport system. This is called secondary active transport:
The transfer of ions downhill drives the transport of other molecules uphill.
- Transport of amino acids/glucose uphill into intestinal cells driven by the co-transport of sodium in the same direction, but downhill. Then sodium extruded by an ATP-driven pump.
The intestinal co-transport of glucose and sodium is exploited in the rehydration of patients with cholera.

Macromolecule Molecule transportation across membranes

Giving Out and Taking In

Small molecules (sugars, AAs, ions) are transported actively & passively.
Cells transport macromolecules / particles (proteins, bacterial cells) across membranes by:
- Exocytosis: macromolecules secreted in vesicles, fusing with the plasma membrane to release contents outside, that doesn't mix with other components of the cytosol.
- Endocytosis: substances (macromolecules / particles) are taken in by the cell through plasma membrane region enclosing it in a vesicle.
Both exocytosis and endocytosis depend on membrane fluidity, which is the ability of membrane components to move about.

Exocytosis

May be constitutive (happens all the time) or regulated (only when needed).
Constitutive:
- Eukaryotic cells: transport vesicles constantly shuttle newly made lipids/proteins from the Golgi body to the plasma membrane.
- Secrete various types of molecules constantly (fibroblasts secrete the large protein procollagen, which forms the collagen of connective tissue).
Regulated:
- Proteins secretion such as digestive enzymes (pepsin, amylase, lipase) / small molecules are stored in secretory vesicles that fuse with the plasma membrane in response to an outside signals such as when signaling to endocrine glands secretion of peptide hormones (insulin / glucagon).
During exocytosis, the vesicle membrane becomes incorporated into the plasma membrane. The membrane proteins and lipids are later recovered by endocytosis and incorporated into new secretory vesicles, so the surface area remains relatively unchanged.

Endocytosis

Two types, differing in vesicle size:
- Pinocytosis (cell drinking): fluid and small particles are taken into small vesicles ( $∼150 nm$ in diameter).
- Phagocytosis (cell eating): large particles (micro-organisms & cell debris) are taken into large vesicles/vacuoles ( $∼250 nm$ in diameter).
Most cells carry out pinocytosis.
The captured particles transported where phagocytic vesicles (phagosomes) fuse directly with lysosomes.
Pinocytic vesicles end up in endosomes (unless specifically retrieved) that are transfered from where they are taken to lysosomes. After that, the breakdown products of lysosomal enzyme action are transported across the lysosomal membrane into the cytosol for reuse.

Receptor mediated Endocytosis

Specific macromolecules enter cells via receptor-mediated endocytosis.
Receptor–hormone complexes cluster in coated pits (dents with clathrin protein coating on the cytosolic side).
Endocytosis begins with invagination of clathrin coated pit containing a cluster of receptors to be engulfed. The clathrin forms a lattice around it, which causes the formation of a coated vesicle about 80 nm in diameter.
The vesicle rapidly loses its clathrin coat, which returns to the plasma membrane to form a new coated pit. The ‘bare’ vesicle fuses with an endosome whose acidic interior (maintained by ATP-driven proton pumps) causes most protein–receptor complexes to dissociate. This sorting enables the proteins and the receptors to be directed to different destinations.
Vesicles containing the receptors bud off from the endosomes and are returned to the plasma membrane. Proteins are taken to the lysosomes, where they are broken down and released into the cytosol.
An example is the animal uptake of low density lipoprotien (LDL) cholesterol needed for membranes / steroid hormones. If the cells need cholesterol, they make LDL receptors which cluster in coated pits on the plasma membrane taken into the cell and returned. whereas the receptor is recycled, the LDL will be taken to lysosomes for degradation to realease cholesterol for reuse.

Membrane Fusion

Exocytosis & endocytosis both involve fusion of two separate membrane bilayers.
Fusion process:
- Two layers come close (bilayer adherence).
- Surface water must be removed from the bilayer surfaces (destabilization).
- Lipids of the two bilayers mix (bilayer joining).
Fusion is possibly be catalysed by special proteins (fusigenic proteins). Occurs not only in exocytosis & endocytosis but also in cell division / cell fusion & within cells, transport vesicles bud from one organelle and fuse with another

Membranes and Messages

Messages

Lipid permeable membranes helps fat soluable easy crossing, impermeable to charge gradients, membrane protein helps certain crossing, fluid for large molecule crossing to form vesicles.

Messengers

Fat-soluble messengers cross membranes easily such as steroids (cortisol, androgens and oestrogens), vitamins $A$ and $D$ , and thyroxine. Travel in the blood to the liver bearing the message: “Make more glucose!”. Diffuse across the plasma membrane of liver cells & bind to receptors in the cytosol changing the receptor shape & synthesize glucose.
Water-soluble messengers is not as the above but as adrenaline and insulin cannot. Bind to receptor on the cell surface (integral protein with 7 α -helical regions with a membrane on the outer surface that recognize messenger facing cytosol that bears a site that recognizes GTP-binding protein).

GTP binding Proteins

In interaction with the messangers in the receptor result in shape change and G-protein interaction. In active it has GTP which has 3 subunits bonded to the inner monolayer of plasma.

Second messengers

G-proteins activates an enzyme which produces a second messenger molecule. The enzyme is adenylate cyclase and produces the second messenger cyclic adenosine monophosphate (cAMP) from ATP. The second messenger triggers a series of enzyme-catalysed reactions which, among other effects, may bring about phosphorylation of ion channels in the membrane, resulting in their opening.
Cyclic AMP causes smell, tastes and sight through channel operation.

Synapses

Nerve cells have $70 mV$ in a small depolarized region that generates actonal nerver impulse to the brain due to ATP-driven sodium pump.
When an impulse gets to the axon end has gap synanse between the next nerve or msucle which is $50 nm$ is transmited with fluid membranes property from membrane fuse with neurotransmitter vesicle to reach next brain regeion

Studing membrane Beetroot

Studied by using beetroot, pigment indicates rupture to test various membrane stabilities.